The present invention relates generally to commercial apparatus for solvent extraction of oils and soluble materials from solids by use of volatile liquid hydrocarbon solvents. More particularly, our invention relates to apparatus in which high tonnages of extract-bearing solid material, such as tar sand, for example, may be processed in a continuous manner, wherein the material is moved substantially in a horizontal plane around a circular pathway as the solvent percolates through a slurry of the solid material while confined in individual treatment cells.
The art of solvent extraction for the recovery of bitumen from oil or tar sand is known, as exemplified by the apparatus and processes disclosed in U.S. Pat. Nos. 4,448,667; 4,422,901; and 4,221,764, which are owned by the assignee of the present application and which are incorporated herein by reference. It is thus, well known that oil or bitumen can be extracted from tar sands with a solvent, generally a volatile hydrocarbon or combinations of volatile hot hydrocarbons to provide a crude product for subsequent cracking in petroleum refineries. The solvent extraction of bitumen from tar sand is easily accomplished under laboratory conditions; however, realization of a commercial apparatus to date has not been achieved. Any apparatus, to be commercially feasible, must be capable of effectively removing the bitumen from many thousands of tons of sand per day to be economically profitable. In addition, such a large scale commercial apparatus must be safe and environmentally acceptable which is also difficult to realize when dealing with great volumes of volatile solvent materials.
It has been proposed, in the above-referenced patents, to provide a commercial solvent extraction system utilizing a chain of basket conveyors having parallel, endless chains arranged with upper and lower runs that extend in a generally horizontal direction. The chains carry a succession of spaced-apart process baskets with perforated bottoms extending crosswise between them. The baskets are pivotally connected with the chains at each end such that when the conveyor moves downwardly from the upper run to the lower run, the baskets remain upright. When the baskets travel upwardly from the lower run to the upper run, they are inverted to dump their contents and then restored to the upright position for loading with a slurry of tar sand and solvent to begin the horizontal, upper run. Trays are spaced beneath the upper and lower runs to collect and dispose of the solvent and extract that drains from the baskets. This prior extractor apparatus is unsatisfactory for high tonnage commercial plants because it would not be economically feasible to construct due to the size and height of the required structure. It would also be quite difficult and expensive to achieve proper sealing of the volatile solvents within such apparatus.
Solvent extraction in the production of vegetable oils from seeds, as for example soybeans, cottonseed, and the like, has been accomplished in apparatus disclosed in U.S. Pat. No. 2,840,459, which is also owned by the assignee of the present application. The rotary cell-type solvent extractor apparatus disclosed therein, having the trade name "Rotocel", performs effectively for small capacities on the order of two to three thousand tons per day of vegetable seeds. The device utilizes a bottom dump feature wherein the perforated bottom floor of each cell hinges downwardly to dump the vegetable seeds after processing. This prior apparatus has not found use in the solvent extraction of bitumen from tar sands because vegetable seeds and tar sands differ greatly in their respective physical properties. It is desirable in solvent treating tar sands, to utilize a fine mesh screen at the bottom of the treatment cells in order to keep the fines from entering the oil recovery system. Tar sand is a very abrasive material which may cause wear and premature failure of a relatively fragile, fine mesh screen if it were utilized in the bottom dump type of discharge present in the Rotocel device. The material must slide over the screened bottom of the cell during the discharge operation in that device. A commercial plant for tar sand processing, in order to be feasible, must be capable of efficiently, safely and economically processing on the order of fifty-thousand tons or more of sand every 24 hours, whereas a large oil seed plant process employing the Rotocel apparatus has a throughput of only about 3000 tons per day. The capacity of this known device is, thus, limited and not suited for tar sand. Ideally, in a commercial solvent extraction plant for the processing of tar sands, the apparatus should have a large throughput capacity and also be capable of utilizing potentially corrosive and volatile solvents and handling abrasive solids.
Our invention provides a circular solvent extractor apparatus which is suitable for use in a high-tonnage commercial plant for the treatment of tar sands. The circular solvent extractor of the present invention includes a plurality of individual batch-type cells which permits the recharging of solvent at discrete points along the machine while locating the key mechanical components such as wheels, bearings and drive components outside of the solvent vapor and abrasive particle environment of the process. The invention further provides a feed system for the slurry which will insure development of a homogeneous bed across the machine. Still further, our invention provides a rapid, 180.degree. inversion dumping system which permits the use of fine mesh screens in the bottoms of the treatment cells. The fine mesh screens permit percolation of the solvent therethrough without contamination by solid fines. Since the cells are completely inverted in the discharge zone, abrasive wear of the fine screen material is minimized and the cell bottom life is extended. Our invention further provides a circular solvent extractor in which the process zone, containing streams of volatile liquid hydrocarbon solvents, is hermetically sealed from the atmosphere to prevent the escape of solvent vapors therefrom. Thus, our device represents a significant advance in improving the safety, environmental acceptability and economy of the process by greatly minimizing solvent vapor losses from the extractor apparatus. The solvent extractor of our invention is also thermally insulated so as to minimize the size of the solvent vapor emergency venting system. Still further, our invention provides a circular solvent extractor in which the inner and outer rotating sidewalls of the machine are structurally interconnected by spokes which also function as stream splitters between adjacent cells.